1. Field of the Invention
The present invention relates to a method of reading and processing images such as those recorded or photographed on microfilms by using a line sensor.
2. Related Art
Conventionally, there is provided a microfilm scanner for reading images of microfilms by using an image sensor such as a line sensor. By reading images with the image sensor, the read images can be processed as digital image signals, which makes it easier to output them to a printer, to store them in an optical magnetic disk, or to transfer them to another image processor.
For this type of a microfilm scanner, it is known to change a magnification (or scale factor) of output images by changing an image density of images read by a line sensor, whereas a magnifying power of an optical system is fixed, since it makes construction of the optical system simpler so as to achieve significant compaction and weight savings.
Since an original is reduced in scale generally to one-tenth (1/10) through one-fiftieth (1/50) to be photographed on microfilms, the photographed image is expanded to 10 through 50 times the size of the microfilm image when outputting images after being read. The output image size is predetermined, for example, for printing and for displaying on a CRT display. Therefore, it is required that the images be fitted to the predetermined or fixed size before being output and that the magnifications or scale factors should be changed in consideration with a frame size of the films.
For changing an output magnification ratio, the output image size can be reduced by intermittently deleting pixel signals read by the line sensor along the main and subsidiary direction at a certain ratio. In addition, it can be expanded by repeating a portion or portions of identical pixel signals along the main and subsidiary direction at a certain ratio. If the image size is reduced to a scale smaller than that of a magnification (scale factor) at which all pixels of the line sensor can be output without deleting nor repeating (hereinafter, "reference magnification or scale factor"), a spacing from a portion of the projected image to the next portion of the projected image, both portions being read by the line sensor, must be expanded in comparison with that for the reference magnification.
Conventionally, however, an image reading or scanning speed of the line sensor is always fixed, therefore, image areas unnecessary for outputting their image signals are also read though they will not be used for an output of the images when read images is reduced to a size of a magnification ratio which is smaller than the reference magnification ratio. It results in a wasteful long-time reading of the images. Accordingly, to solve this problem, there is a way to extend a read spacing in a subsidiary scanning direction by increasing a moving speed of the line sensor. In other words, the distance between the adjacent main scanning lines are extended.
Although the moving speed of the line sensor is increased as the reduction ratio becomes greater, there is a limitation on increasing the moving speed since a stepping motor generally used for moving a line sensor also has a limit on a driving speed.